Islet and pancreas transplantation for diabetes currently requires chronic immunosuppression. Even with immunosuppression, however, recurrent autoimmunity can destroy beta cell allografts when grafts are transplanted into recipients with Type 1 diabetes. We believe that the most promising alterative to current strategies is the induction of immunological tolerance to islet grafts. This Project focuses on the role of CD154 (CD40 ligand) in allogeneic hemopoietic tolerance. CD154 plays a central role in T cell activation; facilitates long term islet (and skin) allograft survival in non- autoimmune mice and in non-human primates; and prevents graft versus host disease (GVHD) in hematopoietic chimeras. We have generated preliminary data showing that treatment with anti-CD154 mAb reduces myeloablative-conditioning requirements and prevents GVHD in a allogeneic bone marrow recipients. These hemopoietic chimeras readily accept donor-specific skin grafts. Most importantly, allogeneic hemopoietic chimerism in diabetic NOD mice is shown to preclude recurrent autoimmunity and permit long-term survival of allogeneic islets. Additionally, preliminary data also document that infection with the non- cytopathic lymphocyte choriomeningitis virus (LCMV) at the time of bone marrow transplantation abrogates tolerance, leading to failure of bone marrow engraftment and death. Infection two weeks after bone marrow transplantation did not abrogate chimerism and was not lethal. The goal of this project is to analyze and understand these very promising protocols in detail and facilitate their translation into clinical medicine. Specific Aim No.1 is to test the hypothesis that mixed hemopoietic chimerism induced by sub-lethal irradiation and anti-CD154 mAb treatment in diabetic autoimmune NOD mice will prevent destruction of donor-specific islet allografts by recurrent autoimmunity. Specific Aim No. 2 is to understand how virus infection perturbs bone marrow transplantation and subsequent donor-specific allograft tolerance in autoimmune diabetic NOD mice. Specific Aim No. 3 is to define the mechanisms of tolerance that permit the creation of allogeneic hemopoietic chimerism in autoimmune diabetic NOD recipients and that fail in the presence of viral infection.